R-125 is useful as an alternative to regulated freons and is used for refrigerant and so on. Known processes for producing R-125 are carried out in the presence of a chromium oxide catalyst resulting from metathesis of ammonium dichromate (U.S. Pat. No. 5,334,787 and U.S. Pat. No. 5,399,549).
In the above-mentioned conventional processes, a considerable amount of R-115 is produced as a by product. The amount of R-115 as a by-product increases as the production of R-125 is increased, and in some cases, amounts to several % based on R-125.
R-115 is a target of the regulations by the ozone layer protection law, and not only its production as a by-product is unfavorable but also a considerable amount of R-115 in R-125 adversely affects the performance of R-125 as a refrigerant by allowing plating of copper eluted from refrigerant pipes. Therefore, in general, it is preferred to reduce the amount of R-115 as a by-product to at most 1,000 ppm, based on R-125.
It is known that a mixture of R-125 and R-115 is azeotropic when R-125/R-115=79 wt %/21 wt % (U.S. Pat. No. 3,505,233). Also, mixtures of R-125 and R-115 containing at least 99 wt % of R-125 are virtually azeotropic. Therefore, R-125 and R-115 are quite difficult to separate by distillation although they have such different boiling points as -48.5.degree. C. and -39.1.degree. C.
In order to solve the difficulty in separation, various methods have been proposed. However, many of them requires cumbersome operations such as extractive distillation, adsorptive removal, selective reduction of R-115 and selective fluorination of R-115. As a solution to the problem in production of R-125, a catalyst which suppresses production of R-115 is demanded.
JP-A-9-511515 discloses an attempt to decrease R-115 through improvement of a catalyst in which a hydrofluorochloroethane of the general formula C.sub.2 HCl.sub.1+x F.sub.1+y (wherein each of x and y is independently 0, 1, 2 or 3, and x+y=3) is fluorinated in the presence of a fluorination catalyst containing "zinc or a zinc compound" and "chromium fluoride or chromium oxyfluoride". However, the effects of decreasing R-115 as a by-product in relation to R-125 is not satisfactory.
Further, production of R-125 using a conventional fluorination catalyst has a drawback that the life of the catalyst is relatively short in the case of vapor phase fluorination.
As a method of extending the life of a catalyst, incorporation of chlorine gas or oxygen gas in the reaction gas is generally known. Incorporation of chlorine furthers fluorination of R-125 to produce R-115 as a by-product. On the other hand, incorporation of oxygen leads to formation of chlorine and water through oxy reaction of hydrogen chloride produced as a by-product on a chromium catalyst. Therefore, incorporation of oxygen, like incorporation of chlorine gas, results in a highly corrosive atmosphere coupled with hydrogen chloride and hydrogen fluoride, because not only R-115 but also water is produced as a by product.
For this reason, there is demand for a catalyst which has a long life, when supplied with a reactant gas directly involved in the reaction, in the absence of an additional gas such as chlorine gas and oxygen gas. If the catalyst has a high activity, it is possible to reduce the amount of the catalyst and reduce the size of a reactor which requires an expensive material of a nickel type.
Further, use of a combustion gas combustible under quite various conditions such as hydrogen in preparation or activation of a catalyst is reported (U.S. Pat. No. 5,494,873), but because such use of a combustion gas on an industrial scale requires an extra investment for safety, it is preferred to use an inert gas such as nitrogen only, in addition to the reactant gas. Further, use of such an inert gas can lower the yield of the desired product.